The influence of grain size on the fraction of transgranular fracture in polycrystalline ceramics was investigated with a stochastic model. Crack propagation path was divided into three cases according to the arrival position of a crack and the angle between a main crack and a cleavage plane. The probability that a crack propagates transgranularly or intergranularly was derived from the above model with stochastic calculation. The influence of grain size on the fraction of transgranular fracture was formulated as a function of grain size. This stochastic model could well explain the influence of grain size on the fraction of transgranular fracture in polycrystalline Al2O3 reported previously.